Numerous communications applications, such as cellular and personal communications services (PCS), as well as multi-channel multi-point distribution systems (MMDS) and local multi-point distribution systems (LMDS), conventionally receive and retransmit signals from subscribers utilizing antennas mounted at the tops of towers or other structures. Other communications systems such as wireless local loop (WLL), specialized mobile radio (SMR), and wireless local area network (WLAN), have signal transmission infrastructure for receiving and transmitting communications between system subscribers that similarly utilize various forms of antennas and transceivers.
All of these communications systems require amplification of the signals being transmitted by the antennas. For this purpose, it has heretofore been the practice to use a conventional linear power amplifier system placed at the bottom of the tower or other structure upon which the antennas are mounted. From the base of the tower, the conventional linear power amplifier system typically couples to the antenna elements mounted on the tower with coaxial cables. Coaxial cables, however, introduce power losses that are proportional to length. To overcome these power losses, substantial amplification is typically required, which necessitates the use of more expensive, higher power amplifiers.
Moreover, the diameter of the cables must generally be of a low loss variety to mitigate insertion losses. In addition to increasing system material costs, the low loss cables characteristically have large diameter cross-sections. Thus, along with the relatively long length of cable required by the system configuration, the large diameter of the cables can contribute towards making a system vulnerable to damage sustained from high wind conditions. That is, the dimensions of the cables increase the wind friction experienced by the system.
The size and number of coaxial cables further require reinforcement of the tower structure to accommodate loading forces associated with the weight of the cables. System architects may consequently implement costly preventative design features and expect periodic cable disconnections and other repairs.
As discussed herein, insertion losses associated with the cables may necessitate some increases in the power amplification. A ground level infrastructure or base station typically provides the compensatory amplification, thus further increasing the expense per unit or cost per watt. Of note, output power levels for infrastructure (base station) applications in many of the foregoing communications systems are typically in excess of ten watts, and often up to hundreds of watts, which results in a relatively high effective isotropic power requirement (EIPR).
For example, for a typical base station with a twenty-watt power output (at ground level), the power delivered to the antenna, minus cable losses, is around ten watts. In this case, half of the power has been consumed in cable loss/heat. Such systems require complex linear amplifier components cascaded into high power circuits to achieve the required linearity at the higher output power. Typically, for such high power systems or amplifiers, additional high power dividers must be employed. Operating characteristics of such divider equipment may introduce further insertion losses associated with the equipment, itself.
Some of such losses are addressed in certain instances by positioning amplification equipment closer to the antenna(s) on the tower mast. While helpful in mitigating some insertion losses associated with cables running up the towers to the antenna(s), such placement of the amplifiers still fails to address insertion losses associated with the jumper cable that connects the amplifier to the antenna, as well as any power divider disposed therebetween. Moreover, even where an antenna has multiple elements, those elements are typically coupled to and serviced by a common amplifier and divider. Thus, failure of a single amplifier, divider or other amplifying component may effectively render the entire system inoperable. In this manner, the reliability of a system having multiple elements remains undermined by the collective dependence of the respective elements on common components. Furthermore, the relative inaccessibility of the amplification equipment attributable to its proximity to the to the tower mast can compound repairs and other maintenance. Consequently, inefficiencies associated with insertion losses continue to hinder operation and result in a relatively high cost of unit per watt.